What Is PCB Manufacturing…Really
Thomas Smiley, Precisionpcbs
First. We have to make a distinction between an unpopulated bare green printed circuit board and an assembled (populated), functional, printed circuit board. Populated boards in the industry are referred to as assemblies, or PCBA’s. Everything below references bare boards (PCB’s).
Second. The function of the bare board is to provide an efficient and fast way to interconnect all of the components that allow the system to do something useful or fun. It is the canvas, the base, the arteries and veins. You get the picture.
What is PCB manufacturing? We hear the following a lot:
Newbie customer asks “…so, to make a printed circuit board, you just pour some green fiberglass and copper into your machine, load my files, and our boards come out at the end of the machine…right?” Wrong.
I have often thought it should be that easy. Like the photo print machines in the drug store. Very Old School, you sent the exposed roll of film to a processing lab, then a couple of weeks later, picked them up at the drug store. Becoming Old School, you bring your flash drive to the drug store, load it in their viewer, crop and fix your red eye, hit print and get your photos in minutes. Current school, who needs prints? Unlikely that printed circuit boards will follow this path.
The real deal is that there are 15 to 25 steps, involving multiple technologies, from your Gerber files to a finished board. For the short version and a photo walk through, see www.precisionpcbs.com/pcb-for-newbies/ ). For the full walk through, read on.
Today’s printed circuit board is constructed of alternating layers of fire resistant fiberglass resin having thin paths ( .005” w x .00135” thick ) (traces) of copper on the surface making the connections between various components that are mounted on the top and bottom of the board at a later process.
The beginning point of a circuit board is a sheet of this fiberglass which has thin copper covering the entire top and bottom surface….this is background info that you need to know.
Between the layers with the copper paths are insulating layers of the same fiberglass resin. Some boards have only a top and bottom, called double sided boards, and some have multiple layers like a 7 layer dip. Layer count can be up to 30 layers, but most are between 4 and 12. These inner layers are connected to one another with holes at specified locations that are filled with copper. These are called vias. These vias are connected to the inner layers during the plating process.
Which brings up how the vias get there in the first place. Drilling. From your Gerber file package is another file called the tool file or drill file. This tells us where all of the holes need to be. So, we stack 2 or 3 raw copper panels on a big CNC machine, load the drill program and simultaneously drill usually 4 or 5 stacks of panels to the required x,y coordinates.
In the case of multilayer boards, we prepare the inner layers first ( reference the precision etching described below ), then create a “book” of copper layers alternated with glue we call “prepreg”, and press the book into one solid laminated board.
So back to the copper paths. The beginning point of a circuit board is a sheet of this fiberglass which has thin copper covering the entire top and bottom surface. Somehow we have to take this and make the maze like paths to carry the signals. We do this by precision etching the copper away. Think of it as a subtractive process, like sculpture. ( the artist said he carved the image of the lady out of stone by removing all of the stone that did not look like a lady…simple right?). This is where your electronic Gerber files come in to play.
Your files are used to create photographic film with these images of the paths. Once the film is created it is contact printed onto the copper/fiberglass panel. The image is actually placed over a special photo sensitive material called “resist”. Why is resist needed? Because after developing, it covers only the copper paths we want and RESISTS ( duh) the chemical etchant. This process is so precise that we can control the width of the paths to .001” in some cases. Typical is .005”. New technologies are coming on line that stream line the contact printing process, but in today’s world 95% of boards produced use the above 65 year old process. ( Crazy the process has lasted this long ).
Once the boards have the correct copper paths ( traces ), they are ready for their epoxy coating, which is basically painted on or pushed on with a squeegee. This coating gets baked on for durability.
Why are PCBs green? Nobody knows why green is the norm, and increasingly, other colors are used like blue ( popular with gold finishes), red, black, white, and sometimes purple. The important thing is that this green finish is actually a coating of insulating epoxy over the copper paths to prevent inadvertent shorting with other parts or equipment and to slow down moisture from being drawn into the fiberglass.
Next step is applying a metallic coating to the exposed mounting pads which will not just promote better soldering, but prevent the bare copper from oxidizing with exposure to the atmosphere. Old School is solder made from a combination of tin and lead. This works really well, but has 2 problems. First, lead is a bad substance to have around and has been banned from most operations. We have now gone to an alloy with copper and tin and we call it lead/free solder. Second problem is that this solder coating is thick and tends to create a slight “pillow” in the middle of the pad. With today’s very very very small parts, the pillow can prevent the parts from mounting flat on the board surface.
In the last 5 years, the industry has moved to a combination of super thin gold over thicker nickel.
Why gold over nickel? For the full explanation go to:
http://www.pcbdesignschool.com/2012/11/25/when-enig-doesnt-solder/.
The short answer is
1. The pads will be completely flat so the parts sit nicely on the board.
2. Nickel, if un-corroded, is an excellent surface to solder to and gold protects the nickel from corrosion. The gold is so thin (2-3 microns ) that it burns off in the solder process.
Now, we are ready for the printing on the board. Ever notice that there are some funny writings on the green boards? These are typically to identify the specific location for a specific component. This is an aid during pcb assembly and should a repair be necessary, allows the technician to lookup the exact part to be replaced. Further, the manufacturer will put their identifying mark on the board along with a date of manufacturer, usually in week/year. Most often the boards are approved by U.L. who also has a marking system that is on the board. The printing is done either with a silkscreen method like a tee shirt, or a photographic process similar to the contact printing process described for the circuit paths.
The last manufacturing process is cut to shape. Remember, these boards are made from larger panels. Your final board will be smaller and we make many on a single panel. So, we stack the boards usually 3 high on a large CNC routing machine and cut away.
Final step is testing. Each board is subjected to an electrical test to verify that all of the little copper paths are correctly connected to one another in the correct way. Now we are done.
For a peek at the machinery involved getting all of this done follow this link and scroll to the bottom of the page: www.precisionpcbs.com/bare-board
Final comment. The industry is so sophisticated, and so competitive, that most credible suppliers try to make it look as easy as pouring fiberglass and copper into a machine. Like the duck in the pond, calm and collected above the water and paddling like crazy below the surface. Quack, Quack.